1. Field of the Invention
This invention relates to a probe to be used for high density information recording/reproduction by applying the principle of the scanning probe microscope (SPM). It also relates to an information recording/reproduction apparatus comprising such a probe.
2. Related Background Art
The scanning tunnelling microscope (STM) and the atomic force microscope (AFM) are two typical types of microscopes that can be used to observe the surface condition of a specimen with a level of resolution useful for seeing atoms.
The operation of STM is based on the following principle. As a sensing needle electrode that is a metal needle is brought close to the surface of an electroconductive specimen to make them separated from each other by a distance less than tens of angstroms and a voltage is applied between the sensing needle electrode and the specimen, a tunnelling current flows therebetween. The tunnelling current sensitively reacts to the distance between the sensing needle electrode and the surface of the specimen. In the STM, a feedback control mechanism is activated to regulate the distance between the sensing needle electrode and the surface of the specimen in order to maintain the tunnelling current to a constant level, while the sensing needle is driven to two-dimensionally scan the surface of the specimen. The locus of the tip of the sensing needle can be imaged by mapping the feedback control signal in synchronism with the two-dimensional scanning operation while maintaining the tunnelling current at the constant level. If the physical properties of the specimen are invariable on the entire surface thereof, the imaged locus can be regarded as reflection of the surface profile of the specimen. A piezoelectric element is normally used to regulate the distance between the sensing needle electrode and the surface of the specimen and the scanning operation of the sensing needle electrode.
On the otherhand, the operation of an AFM is based on the following principle. As a sensing needle having a sharp tip is brought very close to the surface of a specimen, the tip of the sensing needle is subjected to the force of atoms on the surface of the specimen. If the sensing needle is supported by an elastic body, the atomic force that the tip of the sensing needle receives from the surface of the specimen can be expressed in terms of the displacement of the elastic body. In the AFM, a feedback control mechanism is activated to regulate the distance between the sensing needle and the surface of the specimen in order to maintain the displacement at a constant level, while the sensing needle is driven to two-dimensionally scan the surface of the specimen. The surface profile of the specimen can be imaged by mapping the feedback control signal in synchronism with the two-dimensional scanning operation. A cantilever is normally used for the elastic body. A cantilever is usually displaced only to a very small extent and the displacement is detected by shooting the cantilever from the back by means of a laser beam and detecting the displacement by way of the angle of deflection of the reflected beam or by arranging an STM behind the cantilever and detecting changes in the tunnelling current of the STM. In the case of the AFM, a contact mode of operation of driving the sensing needle that is held in contact with the surface of the specimen by a small force is feasible so that the surface profile of the specimen can be imaged directly from the displacement of the elastic body (the deflection of the cantilever). In other words, in a contact mode of operation, the distance between the sensing needle and the surface of the specimen is constantly nil and hence does not require the use of a feedback control mechanism so that the surface can be observed by means of a high speed scanning operation. If an electroconductive sensing needle electrode is used for the sensing needle of the AFM, it is possible to observe the distribution of electric conductivity on the surface of the specimen while observing the surface profile.
Both the STM and the AFM are characterized in that they can be used to directly observe the surface of a specimen by utilizing the interaction of the tip of the sensing needle and the surface of the specimen that are held very close to each other. Various microscopes have been developed on the basis of the interaction other than that of tunnelling current or force. Such microscopes are generically referred to as scanning probe microscopes.
In recent years, information recording/reproduction apparatuses have been developed by utilizing the SPM in order to write information on a minute area of the surface of an object and read the information therefrom. For example, T. R. Albrecht, M. M. Dovek, M. D. Kirk, C. A. Lang and C. F. Quate, Appl. Phys. Lett., 5, 1727 (1989) describes research on applying a pulse voltage of 3 to 8V and 100 xcexcs to a cleavage plane of graphite to produce a hole with diameter of 40 xc3x85 and a depth of 7 xc3x85 there and observing the hole through an STM.
Researches have been made on a technique of densely recording information on a thin film by locally modifying the electric conductivity of the material of the thin film and reproducing the recorded information. For example, K. Takimoto, R. Yano, K. Hatanaka, K. Eguchi and T. Nakagiri, OYO BUTURI 63, 470, (1994) describes a research on applying a pulse voltage to an LB film showing a very low electrodoncutivity by way of a sensing needle electrode to produce on it an area with a diameter of about 10 nm where an electric current easily flows and using it as a bit for recording information.
Similarly, there has been disclosed a technique of recording information by applying a pulse voltage to the SiN/SiO2 interface of an SiN/SiO2/Si multilayer structure by means of a sensing needle electrode and implanting and storing an electric charge there and reading the recorded information by utilizing changes in the electrostatic capacitance there.
Additionally, there is also know a technique of recording information by applying a pulse voltage to a ferroelectric thin film to locally invert the spontaneous polarization of the thin film and reproducing the recorded information by directly detecting the electrostatic force between the site of spontaneous polarization and the sensing needle electrode. With this technique, it is also possible to read the recorded information by detecting changes in the electrostatic capacitance there or the displacement of the medium due to the piezoelectric effect.
These and other techniques are expected to make it possible to record information very densely.
When reading the information recorded by utilizing local changes in the electric conductivity of a recording medium, changes in the electric current flowing between the medium and the sensing needle electrode are detected. In order for the information reproducing operation to be conducted at high speed, it will be desirable to drive an AFM comprising an electrocoductive sensing needle in a contact mode and concurrently detect the electric current flowing between the medium and the sensing needle electrode. In practice, the wire extending from the sensing needle electrode or the medium is led to a current amplifier for current/voltage conversion.
However, the above described known arrangement is accompanied by the following problems.
(1) When the recording density is raised, the area assigned to a recording bit to be written is inevitably reduced. When scanning and reading such a bit at high speed, the total electric charge flowing into the sensing needle from the recording bit will also be reduced. As a result, it is difficult to detect the bit with an enhanced level of contrast. On the other hand, any attempt for improving the bit detecting sensitivity will reduce the speed of reproducing information.
(2) In view of the fact that the stray capacitance produced by the wire connecting the sensing needle electrode or the recording medium and the current amplifier can reduce the bandwidth of the current amplifier, the length of the wire should be minimized to minimize the stray capacitance in order to realize a high speed reproduction of information. However, any attempt for reducing the length of the wire encounters a limit so long as an ordinary current amplifier is used.
On the other hand, in the case of recording information by storing an electric charge or inverting the spontaneous polarization, it is desirable to drive the AFM in a contact mode in order to realize a high speed scanning operation. However, it is highly difficult to directly detect the electrostatic force in a contact mode. In the case of detecting changes in the electrostatic capacitance or in the displacement of the recording medium as a result of piezoelectric effect, it is necessary to detect the response of the electric current or that of the displacement of the elastic body to an application of an AC voltage.
In the former case, the use of a complex detection system is required for lock-in detection and a problem of a reduced bandwidth arises due to a stray capacitance as in the case of directly detecting the electric current (see problem (2) above).
In the latter case, a frequency lower than the resonance frequency of the elastic body has to be used for the AC voltage to be applied, consequently giving rise to a new restriction on the scanning speed to a significant disadvantage for a high speed information reproducing operation.
Therefore, it is an object of the present invention to solve the above identified problems of the known techniques by providing a probe adapted to reproduce information at high speed with an improved bit detection sensitivity even when recording bits are highly densely formed.
Another object of the present invention is to provide an information recording/reproduction apparatus comprising such a probe.
According to an aspect of the invention, the above objects are achieved by providing a probe comprising:
a cantilever having a movable end and formed from an elastic body;
an electroconductive sensing needle arranged at the movable end of the cantilever; and
a field effect transistor arranged at the movable end of the cantilever and having a gate electrode electrically connected to the electroconductive sensing needle.
According to another aspect of the invention, there is also provided a method of manufacturing a probe comprising steps of:
preparing a substrate at least having a semiconductor layer on the surface;
forming a field effect transistor having a gate electrode in the semiconductor layer;
forming an electroconductive sensing needle on the gate electrode; and
processing the substrate to produce a movable end out of the portion thereof where said field effect transistor is formed.